AD883B3 データシートの表示（PDF） - Analog Devices

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AD883B3

Dual, Low Noise, Wideband Variable Gain Amplifiers

Analog Devices 

AD600/AD602

(This system can, of course, be used as an AGC amplifier, in

which the rms value of the input is leveled.) Figure 21 shows the

“decibel” output voltage. More revealing is Figure 22, which

shows that the deviation from the ideal output predicted by

Equation 1 over the input range 80 µV to 500 mV rms is within

450

425

400

375

350

325

300

275

250

225

200

175

150

10µV

100µV 1mV

10mV 100mV

1V

10V

INPUT SIGNAL –V RMS

Figure 20. The RMS Output of A2 Is Held Close to the

“Setpoint” 316 mV for an Input Range of Over 80 dB

5

4

3

2

1

0

–1

–2

–3

–4

–5

10µV

100µV

1mV

10mV 100mV

1V

10V

INPUT SIGNAL – V RMS

Figure 21. The dB Output of Figure 19’s Circuit Is Linear

Over an 80 dB Range

± 0.5 dB, and within ± 1 dB for the 80 dB range from 80 µV to

800 mV. By suitable choice of the input attenuator R1 + R2,

this could be centered to cover any range from 25 mV to 250 mV

to, say, 1 mV to 10 V, with appropriate correction to the value

of VREF. (Note that VSCALE is not affected by the changes in the

range.) The gain ripple of ± 0.2 dB seen in this curve is the re-

sult of the finite interpolation error of the X-AMP. Note that it

occurs with a periodicity of 12 dB—twice the separation be-

tween the tap points (because of the two cascaded stages).

This ripple can be canceled whenever the X-AMP stages are

cascaded by introducing a 3 dB offset between the two pairs of

control voltages. A simple means to achieve this is shown in

Figure 23: the voltages at C1HI and C2HI are “split” by

± 46.875 mV, or ± 1.5 dB. Alternatively, either one of these pins

can be individually offset by 3 dB and a 1.5 dB gain adjustment

made at the input attenuator (R1 + R2).

C1HI

16

A1CM

15

U1

AD600

14 A1OP

VPOS

13

VNEG

12

A2OP

11

10 A2CM

C2HI

9

+6V DEC

C2

–6V DEC 2µF

–6V

DEC

1 VINP

NC 2

3 VNEG

4 CAVG U2

AD636

NC 5 VLOG

NC 6 BFOP

7 BFIN

–6V

DEC

–46.875mV

10kΩ

78.7Ω 78.7Ω

NC = NO CONNECT

+46.875mV

10kΩ

+6V

DEC

3dB OFFSET

MODIFICATION

Figure 23. Reducing the Gain Error Ripple

The error curve shown in Figure 24 demonstrates that over the

central portion of the range the output voltage can be main-

tained very close to the ideal value. The penalty for this modifi-

cation is the higher errors at the extremities of the range. The

next two applications show how three amplifier sections can be

cascaded to extend the nominal conversion range to 120 dB,

with the inclusion of simple LP filters of the type shown in Fig-

ure 15. Very low errors can then be maintained over a 100 dB

range.

2.5

2.0

1.5

1.0

0.5

0

–0.5

–1.0

–1.5

–2.0

–2.5

10µV

100µV 1mV

10mV 100mV

1V

10V

INPUT SIGNAL – V RMS

2.5

2.0

1.5

1.0

0.5

0

–0.5

–1.0

–1.5

–2.0

–2.5

10µV

100µV 1mV

10mV 100mV

1V

10V

INPUT SIGNAL – V RMS

Figure 22. Data from Figure 20 Presented as the Deviation

from the Ideal Output Given in Equation 4

Figure 24. Using the 3 dB Offset Network, the Ripple

Is Reduced

REV. A

–13–

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